Merge branch 'builtin-clean'

Support in-tree builtin module building.

These commits add support for compiling the ZFS module as a built-in
kernel module by copying the module code into the kernel source tree.
Here's the procedure:

  - Create your kernel configuration (`.config` file) as usual. This
    has to be done first so that ZFS's configure script is able to
    detect kernel features correctly.
  - Run `make prepare scripts` inside the kernel source tree.
  - Run `./configure --enable-linux-builtin --with-linux=/usr/src/linux-...`
    inside the ZFS directory.
  - Run `./copy-builtin /usr/src/linux-...` inside the ZFS directory.
  - In the kernel source tree, enable the `CONFIG_ZFS` option (e.g. using
    `make menuconfig`). Note that this option depends on `CONFIG_SPL`
    (see zfsonlinux/spl@744038069d3dc65e721b5b8cc5c37d8c7fcbd8c0).
  - Build the kernel as usual.

ZFS module parameters can be set at boot time using the following syntax
on the kernel command line: `zfs.parameter_name=parameter_value`.

Note that you also need to rebuild the userspace tools (see
zfsonlinux/zfs@f09398cec665259a4c2f96726680fbd3b0a3bac3).

Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Issue #851